A significant problem in the chemical industry is the treatment of wastewater and other process streams to remove pollutants to environmentally acceptable levels. Millions of gallons of wastewater contaminated with heavy metals and other pollutants are generated each day which must be treated to reduce the amount of pollutants to very low levels. For example, particularly toxic pollutants such as lead and mercury must be reduced to 50 ppb and 2 ppb, respectively. The following Table I sets forth a list of "priority" pollutants established under the Clean Water Act and the current federal drinking water and maximum allowable river discharge limits.
TABLE I ______________________________________ EPA Priority Maximum Allowable Pollutant Elements Concentrations (ppm) ______________________________________ Antimony 0.15 Arsenic 0.05 Beryllium 0.000037 Cadmium 0.01 Chromium 0.05 Copper 1.0 Lead 0.05 Mercury 0.002 Nickel 0.013 Selenium 0.01 Silver 0.05 Thallium 0.013 Zinc 5.0 ______________________________________
The need to remove pollutants from large volumes to wastewater to these very low levels has pushed currently available technologies to their limits. Aside from the ability to achieve low levels of contamination, a number of other features are desired in a process for treating wastewater. It is important to be able to treat polluted water in a cost effective manner. In addition to cost effective operation of the process, it is desirable to minimize any required changes to existing water pollution equipment. It is also desirable to reduce the size of a treatment plant as much as possible by increasing the processing rate. In some cases, it is desirable to use a recyclable material to remove the pollutants, so long as the regeneration process does not create more pollution than it eliminates. A wastewater treatment process should create as little solid waste as possible. Finally, the process should not create additional pollution problems such as polluting the treated water with other environmental pollutants.
One of the most popular technologies for treating wastewater is based on a settling process using lime. Calcium hydroxide or magnesium hydroxide is added to the water in a settling tank to absorb the offending contaminant. This technology permits the processing of large volumes without adding polluting chemicals and uses very simple equipment. However, in many cases, the contaminant concentration cannot be reduced low enough to meet EPA standards without using excessive amounts of material and long processing times. Additionally, large amounts of solid hazardous waste in the form of sludge are produced which cannot be effectively regenerated. While landfill has been the most popular means of disposing of sludge, it is rapidly becoming an unacceptable method of handling hazardous waste. Thus, using this technology, the contaminated wastewater problem is essentially being replaced by a solid hazardous waste disposal problem.
Another popular method of cleaning contaminated wastewater is the use of ion exchange resins to filter out the contaminants. Generally speaking, the advantages of ion exchange resins are that it is regenerable, it does not pollute the treated water and usually no separation process is required to remove the ion exchange resin from the treated water. However, the use of ion exchange resins to treat wastewater has a number of disadvantages.
Ion exchange resin processes are slow, very expensive and have low efficiencies. In order to be effective, the wastewater must be passed through a significant amount of resin, usually in the form of a filter bed. This is acceptable for treating small volumes of water to achieve certain levels of purity (e.g., 0.1 ppm of lead). However, as the desired level of purity (e.g. 0.05 ppm of lead) and volume of water increase, this technology becomes increasingly slow or less effective. The complex fabrication process and sophisticated synthetic chemistry involved in developing and producing ion exchange resins significantly contributes to the expense of using ion exchange resins to purify liquid waste and limits the variety of resins available. Ion exchange resin beds may be regenerated, but the wastewater from the regeneration must be treated to remove bulk toxins and then usually passed through the ion exchange resin again to eliminate all the polluted water.
Another technology for removing water soluble material is solvent extraction. This technology is not used in the waste water treatment industry, but rather for reclaiming materials of value such as in the mining industry. In solvent extraction processes, an organic solvent such as kerosene is contacted with the water containing the material to be reclaimed. The organic solvent contains an extractant compound which is preferably highly soluble in the organic phase and significantly less soluble in the aqueous phase. The extractant compound complexes with the material to be removed and the complexed extractant-material remains dissolved in the solvent. The organic and aqueous phases are then separated such as by decanting. The primary advantages of solvent extraction are speed, effectiveness, and ease of regeneration. The extractant compounds are also generally easier to synthesize than ion exchange resins. Therefore, a much broader variety of materials is commercially available and extractants may be tailored to selectively extract particular materials. However, solvent extraction does have disadvantages that make this technology unsuitable for the purification of wastewater.
One major disadvantage is that solvent extraction leaves solvent and extractant residues in the processed water thus creating another pollution problem. The solvents and in some cases the extractants are environmentally toxic. The solvents are generally flammable and toxic which creates an environmental hazard. They may also be expensive thereby contributing to the expense of the process. If regeneration cannot be used, it takes a large volume of solvent to treat a given volume of water and solvent extraction may be prohibitively expensive. While the solvents are easier to regenerate than ion exchange resin and yield a much smaller volume of regeneration waste, the wastes still must be treated, creating yet another pollution problem.
The process of the present invention has the advantages of the above technologies with few disadvantages and is also highly efficient and effective for purifying aqueous solutions. The same equipment that is commonly found in most large scale water and municipal water treatment plants (i.e., settling process equipment) may be used to practice the present process. For a given volume of wastewater, the processing time is generally much less than for ion exchange resins or settling and comparable with processing time for solvent extractions. However, unlike solvent extraction, potentially toxic and flammable solvents are not introduced into the water. Compared to lime settling treatment, very little sludge is produced, yet low contaminant concentrations that meet EPA regulations may be achieved. The getters and carriers used in the present process may be regenerated easily with inexpensive chemicals without producing excessive regeneration wastewater. Because relatively small amounts of sludge are generated and the getters and carriers are not usually expensive compounds, the wastes may optionally be disposed of by landfill or incineration.